This week on HST

HST Programs: January 26 - February 1, 2015

Selected highlights

GO 13314: Characterizing the Elusive Intragroup Medium and Its Role in Galaxy Evolution

Subaru image of the Hickson 40 compact group.

A high proportion of galaxies, perhaps as many as half, are found in relatively sparse, but gravitationally-bound groups of less than 50 members. These systems are generally less than 1-2 Mpcs in extent, with total mass between 1012 and 1013 MSun. The majority of the baryonic material in these systems is believed to lie in diffuse gas in the IntraGroup Medium. Direct detection of that gas is difficult; only the massive groups heat the IGrM sufficiently to excite soft X-ray emission. The present program aims to search for the gas in absorption: the Cosmic Origins Spectrograph will be used to obtain UV spectra of quasars that lie close to galaxy groups; hot gas in the IGrM absorbs the light from the background quasar. Those observations can be used to probe the composition and temperature of gas along the line of sight. The results can be compared against data from other HST programs that survey gas in the outer halos of galaxies, testing how the properties of the IGrM might be influenced by gas escaping from individual galaxies.

GO 13650: The MUSCLES Treasury Survey: Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems

SOHO image of an extremely strong solar flare

M dwarfs - at least, the subset of M dwarfs known as flare stars - are renowned for possessing extremely active chromospheres and coronae. Their discovery as highly variable objects happened largely by chance. Willem Luyten had noticed in 1924 that certain M dwarfs showed spectroscopic variability, with the occasional appearance of emission lines, while in the early 1940s van Maanen commented that two late-type dwarfs, Gl 412B (WX UMa) and Gl 285 (YZ CMi), had brightened by over a magnitude on a handful of parallax plates. The crucial observations came in 1948, when E.F Carpenter noticed that the fainter component of a wide binary system had brightened by more than 3 magnitudes in a matter of minutes. In the succeeding 50 years, these stars have been subjected to extensive observations, particularly at optical and X-ray wavelengths, and the underlying physical processes are relatively well understood. However, most attention has focused on the more active flare stars, and we still have a relatively uncertain grasp on the flare frequency among less active stars. This issue has acquired increased importance with the realisation that somewhere between 10 and 50% of M dwarfs host planetary systems. As the most populous stars in the Galaxy, this also makes M dwarfs the premier planet hosts. The habitable zones in those systems lie much closer to the parent star, and planets are correspondingly vulnerable to detrimental effects from enhanced UV radiation, particularly short-wavelength UV-C. This proposal uses the Cosmic Origins Spectrograph and the Space Telescope Imaging Spectrograph to obtain UV and optical spectra (1150-5700 Angstroms) of nearby M dwarfs, providing a broad sampling of the range of activity levels among these low mass dwarfs.

GO 13665: Exploring the Diversity of Exoplanet Atmospheres in the Super-Earth Regime

Artist's impression of the GJ 1214 system

The first exoplanet, 51 Peg b, was discovered through radial velocity measurements in 1995. 51 Pegb was followed by a trickle, and then a flood of other discoveries, as astronomers realised that there were other solar systems radically different from our own, where "hot jupiters" led to short-period, high-amplitude velocity variations. Then, in 1999, came the inevitable discovery that one of those hot jupiters. HD 209458b, was in an orbit aligned with our line of sight to the star, resulting in transits. Since that date, the number of known transiting exoplanet systems has grown to more than 100 from ground-based observations, most detected through wide-field photometric surveys, while the high-sensitivity data provided by Kepler has added a further 1000+ confirmed systems and ~2000 additional candidates. With the added numbers, observations have pushed detections to lower and lower masses, and it is now clear that the most common type of planet is the "super-Earth" - planets with masses that are several (3-6) times that of Earth and radii 2-4 times larger than Earth. One of the earliest examples is the planet circling the M dwarf, GJ 1214. Such planets have no obvious analogue in the Solar System, and the measured masses and diameters might reflect a range of interior structurees: large rocky bodies with relatively thin atmospheres; dense cores surrounded by a steam atmosphere; or "mini-Neptunes", with rock or ice cores surrounded by extended hydrogen or helium atmospheres. The present program aims to probe the diversity of these systems by using the G141 grism on Wide-Field Camera 3 to obtain time-resolved scanning observations of five transiting systems. The goal is to obtain data that will clearky distinguish between large scaleheight, hydrogen-dominated atmospheres and a more compact, steam-dominated systems.

GO 13679: Europa's Water Vapor Plumes: Systematically Constraining their Abundance and Variability
GO 13829: The ice plumes of Europa

The HST imaging of a potential water plume around Europa's south pole superimposed on an image of the satellite

Europa is the smallest, and the most intriguing, of the four Galilean satellites of Jupiter. With a diameter of 3139 km, Europa is almost twice the size of Earth's moon and significantly larger than Mercury. In 1957, Gerard Kuiper commented that both infrared spectroscopy and the optical colours and albedo suggested that Jovian satellite II (Europa) is covered "by H2O snow". Images taken by the Voyager space probes in the late 1970s (see left) reveal a smooth surface, with only a handful of craters larger than a few kilometres. These features are consistent with a relatively young, icy surface. Subsequent detailed investigations by the Galileo satellite strongly suggest that a substantial body of liquid water, heated by tidal friction, underlies a 5 to 50 km thick icy crust. The presence of this subterranean (subglacial?) ocean clearly makes Europa one of the two most interesting astrobiology targets in the Solar System. Most recently, analysis of observations taken by the Space Telescope imaging Spectrograph (STIS) on Hubble indicated the presence of an extended cloud of Lyman-alpha emission near the polar regions while Europa was furthest in its orbit from Jupiter, strongly suggesting that Europa's oceans may be vaporising into space.Follow-up observations on two further occasions earlier in 2014 failed to detect any emission, suggesting that the emission is either sporadic or periodic; in the latter case, the emission might be related to the location of Europa within its orbit and the consequent tidal strain imposed by Jupiter. The two programs cited here follow up on these observations. GO 13679 is conducting a methodical series of observations designed to image Europa at a progressive series of orbital locations. This program uses STIS to search for H and O auroral emissions at UV wavelengths and will aim to map the distribution of emission at different phases of the Europan orbit. GO 13829 is using UV imaging and spectroscopy with the ACS Solar Blind Camera is searching for fluorescence, with the majority of the observations will be taken while Europa is in eclipse, matched against reference data taken out of eclipse.